1. Field of the Invention
The present invention relates to an optical filter, and more particularly, to a wavelength periodical filter whose wavelength transmission characteristic has periodicity.
2. Description of the Related Art
In a wavelength division multiplexing (WDM) transmission, which is currently a leading optical communication system, a function for freely adding/dropping a certain wavelength signal in part, namely, an optical coupler/splitter plays an important role. Since restrictions are imposed on the amplification band of an optical amplifier used in a WDM transmission, there is a demand for transmitting and processing as many transmission signals as possible in an available wavelength band. At this time, the number of wavelengths and a signal wavelength interval are inversely proportional. Therefore, the valid wavelength band of each channel is narrowed unlimitedly. Accordingly, the transmission characteristic of each channel must be optimized to suppress the influences of the fluctuations of a laser, a temperature change, etc. to a minimum.
For example, a periodical filter is used as a converter for converting, for example, a signal of 100-GHZ frequency spacing to a signal of 200-GHz spacing, or further to a signal of 400-GHz spacing. Periodical filters under research and development at present include modules such as BICS (U.S. Pat. No. 6,130,971) by Avanex Corp. of the U.S., WaveXpander (U.S. Pat. No. 6,160,932) by WaveSplitter Technologies, Inc. of the U.S., Slicer by Chroum Technologies, Inc. of the U.S., Interleaver by JDS-Uniphase Corp. of the U.S., and MGTI (Michelson-Gires-Tournois Interferometer) by Communications Research Laboratory of the Ministry of Post and Telecommunications of Japan. Since these modules trade off an optimization band against a loss, restrictions are imposed on system designs.
Among these modules, modules which have a little insertion loss and a relatively wide optimization band in principle are the BICS by Avanex Corp., and the double GTR (Gires-Tournois Resonator) by the Ministry of Posts and Telecommunications.
Both of the MGTI and the BICS are configured by combining a Michelson interferometer and a GTI (Gires-Tournois Interferometer or a GTR). Such configurations have the most distinguished characteristic of essentially having a band whose transmission characteristic is optimized, because a phase characteristic has an optimization band. The former is configured by attaching a GTR to the tip of an arm of the Michelson interferometer. In this configuration, a GTR is attached to one arm, or both arms.
FIG. 1 shows the configuration of a periodical filter that is configured by attaching GTRs to both arms of a Michelson interferometer.
The configuration, in which the GTRs are attached to both of the arms (this is called a double GTR), has an advantage that an optimization band can be widened to 1.6 times (0.64 nm) in comparison with the configuration where a GTR is attached to one arm (an optimization band is approximately 0.4 nm), by changing the reflectances of the GTRs. However, the lengths of the arms of this interferometer are different, and the interferometer is designed to include as parameters not only the gap between mirrors within a GTR, but also a difference between optical path lengths. Therefore, the lengths of the arms must be controlled with precision of several μs.
Each of these GTRs is configured by a semi-transparent mirror Ma, a 3λ/4 plate, and a total reflection mirror. The semi-transparent mirror Ma and the total reflection mirror are the same as interference filters configuring a normal etalon, and their reflection planes do not have polarization dependence.
FIG. 2 shows the configuration of a BICS.
Since the BICS is designed not to have the difference between optical path lengths, only the gap between mirrors within a GTR may be accurately controlled, which implements the ease of operability.
FIGS. 3A to 3E, and 4 show simulation results of the transmission characteristic of the double GTR.
As shown in FIGS. 3A to 3E, the transmission characteristic of each frequency (channel) is represented by a solid line, and the inclination of the transmission characteristic is represented by a dotted line in the case where the double GTR has different GTR reflectances (8% and 57.2%). These figures show the transmission characteristic in the case where central wavelengths are respectively 1561.42 nm (FIG. 3A), 1553.29 nm (FIG. 3B), 1545.323 nm (FIG. 3C), 1537.61 nm (FIG. 3D), and 1529.55 nm (FIG. 3E) . An optimization band is approximate 0.64 nm in all the cases. Furthermore, FIG. 4 shows the periodical transmission characteristic (indicated by a solid line) in a predetermined frequency range. In this case, isolation is approximately 30 dB or more.
FIGS. 5A to 5E, and 6 show the simulation results of the transmission characteristic of the BICS.
FIGS. 5A to 5E show the transmission characteristic of the BICS at respective frequencies in the case where central wavelengths are respectively 1561.42 nm, 1553.329 nm, 1545.323 nm, 1537.398 nm, and 1529.55 nm, and a GTR reflectance is 17.5%. A solid line indicates the transmission characteristic, whereas a dotted line indicates the inclination of the transmission characteristic. Here, an optimization band is approximately 0.16 nm.
FIG. 6 shows the periodical transmission characteristic of the BICS in a predetermined band. In this case, the isolation of the BICS is approximately 43 dB or more.
A vertical axis in FIGS. 3A to 6 indicates dB, whereas a horizontal axis indicates THz.
Expecting that a WDM communication will be become denser in the future, the wavelength optimization band of the BICS must be significantly widened.